Model-based inspection of shot peened surfaces using fusion techniques

Shot peening is a technique used to increase the flexural fatigue strength of machine parts which are heavily loaded by alternate bending. The impacts of the projectiles induce a compressive strain tangential to the surface which increases its endurance limit. To achieve a defined surface coverage with projectile impacts, this process has to be calibrated by measuring the surface coverage as a function of time. Up to now, this is done visually by inspecting test surfaces with a microscope. Following, the surfaces are compared with a catalog of reference patterns showing different coverage factors. This paper presents a model-based technique enabling an automated inspection of shot peened surfaces. For this purpose, an image series is recorded by varying the surface illumination systematically, and subsequently the series is fused to a symbolic result describing the areas showing shot impacts. Thanks to the simultaneous analysis of signal intensities in illumination space, no consideration of neighboring pixels is necessary to classify each single surface point. However, to assure a consistent result, additional constraints are introduced. Thus, a robust and precise detection of the interesting areas is attained.